Semiconductor device

ABSTRACT

In one embodiment, a semiconductor device includes a semiconductor substrate, and first and second transistors of first and second conductivity types on the substrate. The first transistor includes a first gate electrode on the substrate, a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode, and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region. The second transistor includes a second gate electrode on the substrate, a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode, and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-232490, filed on Nov. 8, 2013, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to a semiconductor device.

BACKGROUND

The currently proposed tunnel FETs (TFETs) can be classified into three types. A first type TFET includes source and channel regions having the same conductivity type and is formed on a bulk semiconductor substrate. A second type TFET includes source and channel regions having opposite conductivity types and is formed on a bulk semiconductor substrate. A third type TFET includes source and channel regions having the same conductivity type and is formed on an SOI substrate. Although such different types of TFETs are proposed, there are few proposals on a circuit including TFETs formed on a semiconductor substrate. However, a TFET circuit has problems different from problems with a MOSFET circuit, and such problems need to be resolved. For example, when the TFETs are electrically connected by an interconnect to form an inverter circuit, source and drain regions of the TFETs on an interconnect side are electrically floating, and therefore a leakage current caused by this needs to be suppressed. Unfortunately, if the TFET circuit is formed by using a structure for resolving such problems, the resulting semiconductor device may have a complicated structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a semiconductor device of a first embodiment;

FIG. 2 is a cross-sectional view showing an example of an interconnect structure of the semiconductor device of the first embodiment;

FIG. 3 is a cross-sectional view showing a structure of a semiconductor device of a first modification of the first embodiment;

FIG. 4 is a cross-sectional view showing a structure of a semiconductor device of a second modification of the first embodiment;

FIGS. 5A to 6B are cross-sectional views showing a method of manufacturing the semiconductor device of the first embodiment;

FIG. 7 is a cross-sectional view showing a structure of a semiconductor device of a second embodiment;

FIGS. 8A and 9B are cross-sectional views showing a method of manufacturing the semiconductor device of the second embodiment;

FIG. 10 is a circuit diagram showing a circuit configuration of a semiconductor device of a third embodiment; and

FIG. 11 is a circuit diagram showing a circuit configuration of a semiconductor device of a fourth embodiment.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanying drawings.

In one embodiment, a semiconductor device includes a semiconductor substrate, a first transistor of a first conductivity type disposed on the semiconductor substrate, and a second transistor of a second conductivity type disposed on the semiconductor substrate. The first transistor includes a first gate electrode disposed on the semiconductor substrate via a first gate insulator, a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode, and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region. The second transistor includes a second gate electrode disposed on the semiconductor substrate via a second gate insulator, a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode, and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region.

(First Embodiment)

FIG. 1 is a cross-sectional view showing a structure of a semiconductor device of a first embodiment.

The semiconductor device of FIG. 1 includes a semiconductor substrate 1, first and second gate insulators 2 a and 2 b, first and second gate electrodes 3 a and 3 b, first and second sidewall insulators 4 a and 4 b, and isolation insulators 5 a to 5 e.

The semiconductor device of FIG. 1 further includes first and second source regions 11 a and 11 b, first and second drain regions 12 a and 12 b, first and second diffusion regions 13 a and 13 b, third and fourth diffusion regions 14 a and 14 b, first and second channel regions 15 a and 15 b, first and second well regions 16 a and 16 b, fifth and sixth diffusion regions 17 a and 17 b, and a semiconductor region 18.

The reference character Tr₁ represents a first transistor formed on the semiconductor substrate 1. The first transistor Tr₁ is an n-type TFET. The n-type is an example of a first conductivity type. The first transistor Tr₁ includes the first gate insulator 2 a, the first gate electrode 3 a, the first source region 11 a, the first drain region 12 a, the first channel region 15 a, the first well region 16 a and the like. The first transistor Tr₁ is a first type TFET in which the first source region 11 a and the first channel region 15 a have the same conductivity type.

The reference character Tr₂ represents a second transistor formed on the semiconductor substrate 1. The second transistor Tr₂ is a p-type TFET. The p-type is an example of a second conductivity type. The second transistor Tr₂ includes the second gate insulator 2 b, the second gate electrode 3 b, the second source region 11 b, the second drain region 12 b, the second channel region 15 b, the second well region 16 b and the like. The second transistor Tr₂ is a second type TFET in which the second source region 11 b and the second channel region 15 b have opposite conductivity types.

The reference character 21 a represents an interconnect for applying a ground voltage (=0 V) to the first source region 11 a. The reference character 21 b represents an interconnect for applying a power supply voltage (=V_(DD)) to the second source region 11 b. In the present embodiment, V_(DD)>0 V. The reference character 22 a represents an interconnect for applying the power supply voltage to the fifth diffusion region 17 a. The reference character 22 b represents an interconnect for applying the power supply voltage to the sixth diffusion region 17 b. The reference character 23 represents an interconnect electrically connecting the first and second drain regions 12 a and 12 b. In the present embodiment, the first and second transistors Tr₁ and Tr₂ are electrically connected by the interconnect 23 to form an inverter circuit.

Hereinafter, each component of the semiconductor device of the present embodiment will be described with reference to FIG. 1.

The semiconductor substrate 1, for example, is a p-type bulk silicon substrate. FIG. 1 shows X and Y directions which are parallel to a surface of the semiconductor substrate 1 and perpendicular to each other, and a Z direction which is perpendicular to the surface of the semiconductor substrate 1. In the description, the +Z direction is referred to as the upward direction, and the −Z direction is referred to as the downward direction. For example, the positional relationship between the semiconductor substrate 1 and the first and second gate electrodes 3 a and 3 b is expressed that the semiconductor substrate 1 is located below the first and second gate electrodes 3 a and 3 b.

The first gate electrode 3 a is formed on the semiconductor substrate 1 via the first gate insulator 2 a. Similarly, the second gate electrode 3 b is formed on the semiconductor substrate 1 via the second gate insulator 2 b. The first and second gate insulators 2 a and 2 b are, for example, silicon oxide films, silicon nitride films, silicon oxynitride films, higk-k films (e.g., HfSiON film) or the like. The first and second gate electrodes 3 a and 3 b are, for example, polysilicon layers, metal layers or the like. Each of the first and second gate electrodes 3 a and 3 b may be a stack film including a polysilicon layer and a metal layer.

The first sidewall insulators 4 a are formed on side surfaces of the first gate electrode 3 a. Similarly, the second sidewall insulators 4 b are formed on side surfaces of the second gate electrode 3 b. The first and second sidewall insulators 4 a and 4 b are, for example, TEOS films, silicon nitride films or the like. Each of the first and second sidewall insulators 4 a and 4 b may be a stack film including a TEOS film and a silicon nitride film.

The isolation insulators 5 a to 5 e are formed on the surface of the semiconductor substrate 1. The isolation insulators 5 a to 5 e are, for example, silicon oxide films. The first transistor Tr₁ is formed on the semiconductor substrate 1 between the isolation insulators 5 a and 5 b. The second transistor Tr₂ is formed on the semiconductor substrate 1 between the isolation insulators 5 c and 5 d. The reference character W₁ represents the width of the isolation insulator 5 c in the X direction.

The first source region 11 a and the first drain region 12 a are formed in the semiconductor substrate 1 so as to sandwich the first gate electrode 3 a. Similarly, the second source region 11 b and the second drain region 12 b are formed in the semiconductor substrate 1 so as to sandwich the second gate electrode 3 b. The first source region 11 a and the second drain region 12 b are p-type regions. The second source region 11 b and the first drain region 12 a are n-type regions.

The first and third diffusion regions 13 a and 14 a are formed in the semiconductor substrate 1 so as to be in contact with the first source region 11 a. Specifically, the first diffusion region 13 a is formed on lower and lateral sides of the third diffusion region 14 a. The first diffusion region 13 a is an n-type region. The third diffusion region 14 a is a p-type region. The third diffusion region 14 a has the same conductivity type as the first source region 11 a and functions as a first source extension region for the first source region 11 a. On the other hand, the first diffusion region 13 a has an opposite conductivity type of the first source region 11 a and the third diffusion region 14 a and functions as a halo region for the first source extension region.

The second and fourth diffusion regions 13 b and 14 b are formed in the semiconductor substrate 1 so as to be in contact with the second source region 11 b. Specifically, the fourth diffusion region 14 b is formed on an upper side of the second diffusion region 13 b. The fourth diffusion region 14 b is a p-type region. The second diffusion region 13 b is an n-type region. The second diffusion region 13 b has the same conductivity type as the second source region 11 b and functions as a second source extension region for the second source region 11 b. On the other hand, the fourth diffusion region 14 b has an opposite conductivity type of the second source region 11 b and the second diffusion region 13 b and functions as a pocket region for the second source region 11 b. In the second transistor Tr₂, a tunneling current flows between the second diffusion region 13 b and the fourth diffusion region 14 b.

The first channel region 15 a is formed between the first source region 11 a and the first drain region 12 a. The first channel region 15 a is a p-type region. Therefore, the first transistor Tr₁ is a first type TFET in which the first source region 11 a and the first channel region 15 a have the same conductivity type.

The second channel region 15 b is formed between the second source region 11 b and the second drain region 12 b. The second channel region 15 b is a p-type region. Therefore, the second transistor Tr₂ is a second type TFET in which the second source region 11 b and the second channel region 15 b have opposite conductivity types. It is noted that the second channel region 15 b has the same conductivity type as the first channel region 15 a.

The first well region 16 a is formed under the first channel region 15 a and is in contact with the first channel region 15 a. The first well region 16 a is an n-type region having an opposite conductivity type of the first channel region 15 a.

The second well region 16 b is formed under the second channel region 15 b and is in contact with the second channel region 15 b. The second well region 16 b is an n-type region having an opposite conductivity type of the second channel region 15 b. It is noted that the second well region 16 b has the same conductivity type as the first well region 16 a.

The fifth diffusion region 17 a is formed on an upper side of the first well region 16 a, and electrically connects the interconnect 22 a on the semiconductor substrate 1 and the first well region 16 a. The fifth diffusion region 17 a is formed between the isolation insulators 5 b and 5 c. In the present embodiment, the fifth diffusion region 17 a is provided to apply a substrate bias voltage (the power supply voltage V_(DD) in this device) to the first well region 16 a. The fifth diffusion region 17 a is an n-type region having the same conductivity type as the first well region 16 a.

The sixth diffusion region 17 b is formed on an upper side of the second well region 16 b, and electrically connects the interconnect 22 b on the semiconductor substrate 1 and the second well region 16 b. The sixth diffusion region 17 b is formed between the isolation insulators 5 d and 5 e. In the present embodiment, the sixth diffusion region 17 b is provided to apply a substrate bias voltage (the power supply voltage V_(DD) in this device) to the second well region 16 b. The sixth diffusion region 17 b is an n-type region having the same conductivity type as the second well region 16 b. It is noted that the sixth diffusion region 17 b has the same conductivity type as the fifth diffusion region 17 a.

In the present embodiment, the second drain region 12 b and the second channel region 15 b are electrically floating. Therefore, when the second transistor Tr₂ is turned on, the second drain region 12 b and the second channel region 15 b have a positive potential. In this case, if the potential of the second well region 16 b is 0 V, a forward bias voltage can be applied to the pn junction between the second channel region 15 b and the second well region 16 b so that a leakage current can flow between these regions.

In the present embodiment, the power supply voltage V_(DD) is therefore applied to the sixth diffusion region 17 b to increase the potential of the second well region 16 b to V_(DD). This makes it possible to keep the potential of the second well region 16 b higher than the potential of the second channel region 15 b. Therefore, according to the present embodiment, a reverse bias voltage can be applied to the pn junction between the second channel region 15 b and the second well region 16 b, so that the leakage current can be prevented from flowing between these regions.

Similarly, in the present embodiment, the power supply voltage V_(DD) is applied to the fifth diffusion region 17 a to increase the potential of the first well region 16 a to V_(DD). This makes it possible to keep the potential of the first well region 16 a higher than the potential of the first channel region 15 a. Therefore, according to the present embodiment, a reverse bias voltage can be applied to the pn junction between the first channel region 15 a and the first well region 16 a, so that the leakage current can be prevented from flowing between the first channel region 15 a and the first well region 16 a.

The semiconductor region 18 exists under the first and second well regions 16 a and 16 b, and is in contact with the first and second well regions 16 a and 16 b. The semiconductor region 18 is a part of the semiconductor substrate 1 where no diffusion region is formed. In the present embodiment, since the semiconductor substrate 1 is a p-type substrate, the semiconductor region 18 is a p-type region.

FIG. 2 is a cross-sectional view showing an example of a interconnect structure of the semiconductor device of the first embodiment.

FIG. 2 shows, in addition to the components of FIG. 1, first and second plug layers 31 and 33, first and second interconnect layers 32 and 34, and first to third inter layer dielectrics 41 to 43.

The first to third inter layer dielectrics 41 to 43 are formed in this order on the semiconductor substrate 1. The first plug layer 31 is formed in the first inter layer dielectric 41. The first interconnect layer 32 is formed on the first inter layer dielectric 41 and covered with the second inter layer dielectric 42. The second plug layer 33 is formed in the second inter layer dielectric 42. The second interconnect layer 34 is formed on the second inter layer dielectric 42 and covered with the third inter layer dielectric 43.

In the present embodiment, the interconnects 21 a, 21 b, 22 a, 22 b and 23 can be formed, for example, by using the first and second plug layers 31 and 33 and the first and second interconnect layers 32 and 34. In the example of FIG. 2, the interconnect 23 connecting the first and second drain regions 12 a and 12 b is formed by using contact plugs in the first plug layer 31, via plugs in the second plug layer 33, and interconnects in the first and second interconnect layers 32 and 34. The semiconductor device of the present embodiment may have an alternative interconnect structure different from that of the example of FIG. 2.

(1) Details of Semiconductor Device of First Embodiment

Details of the semiconductor device of the first embodiment will be described with reference to FIG. 1 again.

In the present embodiment, the first transistor Tr₁ is an n-type TFET of the first type, and the second transistor Tr₂ is a p-type TFET of the second type.

Therefore, the first and second transistors Tr₁ and Tr₂ in the present embodiment have structures similar to each other. Specifically, the first and second channel regions 15 a and 15 b have the same conductivity type. The first and second well regions 16 a and 16 b have the same conductivity type. The fifth and sixth diffusion regions 17 a and 17 b have the same conductivity type. Accordingly, the semiconductor device of the present embodiment has a simple structure in which transistors with similar structures are aligned. In the present embodiment, the substrate bias voltage applied to the first well region 16 a is set equal to the substrate bias voltage applied to the second well region 16 b.

Therefore, according to the present embodiment, the semiconductor device can be simply designed and fabricated. For example, the first and second channel regions 15 a and 15 b can be formed in the same process, which makes it possible to reduce the number of processes for manufacturing the semiconductor device. The same also applies to the first and second well regions 16 a and 16 b and the fifth and sixth diffusion regions 17 a and 17 b.

According to the present embodiment, the structure of the semiconductor device can be made simpler using the structure of first and second modifications described below.

FIG. 3 is a cross-sectional view showing a structure of a semiconductor device of a first modification of the first embodiment.

In FIG. 1, the first and second well regions 16 a and 16 b have the same conductivity type, and the same substrate bias voltage is applied to the first and second well regions 16 a and 16 b. Therefore, the first and second well regions 16 a and 16 b do not always need to be separated from each other.

In FIG. 3, a well region 16 is therefore substituted for the first and second well regions 16 a and 16 b. The well region 16 is formed under the first and second channel regions 15 a and 15 b and is in contact with the first and second channel regions 15 a and 15 b. The well region 16 is an n-type region and has an opposite conductivity type of the first and second channel regions 15 a and 15 b.

In FIG. 1, since the first and second well regions 16 a and 16 b do not always need to be separated from each other, the width of the isolation insulator 5 c between the first and second well regions 16 a and 16 b may be narrowed.

In FIG. 3, the width of the isolation insulator 5 c is therefore reduced to W₂ from W₁. The width of the isolation insulator 5 c, for example, may be set substantially equal to that of other isolation insulators 5 a, 5 b, 5 d and 5 e. According to the present modification, the reduction of the width of the isolation insulator 5 c makes it possible to increase the degree of integration of the semiconductor device.

FIG. 4 is a cross-sectional view showing a structure of a semiconductor device of a second modification of the first embodiment.

When the first and second well regions 16 a and 16 b of FIG. 1 are replaced by the well region 16, any one of the fifth and sixth diffusion regions 17 a and 17 b may be deleted.

In FIG. 4, a diffusion region 17 is therefore substituted for the fifth and sixth diffusion regions 17 a and 17 b. The diffusion region 17 is formed on an upper side of the well region 16 and electrically connects the interconnect 22 on the semiconductor substrate 1 and the well region 16. The diffusion region 17 is formed between the isolation insulators 5 d and 5 e. In the present modification, the diffusion region 17 is provided to apply a substrate bias voltage (the power supply voltage V_(DD) in this device) to the well region 16. The diffusion region 17 is an n-type region having the same conductivity type as the well region 16. According to the present modification, the area of the diffusion region for the substrate bias can be reduced, which makes it possible to increase the degree of integration of the semiconductor device.

When the fifth and sixth diffusion regions 17 a and 17 b of FIG. 1 are replaced by the diffusion region 17, any one of the isolation insulators 5 b and 5 c may also be deleted.

In FIG. 4, an isolation insulator 5 f is therefore substituted for the isolation insulators 5 b and 5 c. The isolation insulator 5 f is formed between the first and second drain regions 12 a and 12 b and is in contact with both the first and second drain regions 12 a and 12 b. Specifically, the first and second drain regions 12 a and 12 b of FIG. 4 are in contact with the same isolation insulator 5 f. According to the present modification, the area required for the isolation can be reduced, which makes it possible to increase the degree of integration of the semiconductor device.

Hereinafter, details of the semiconductor device of the first embodiment will be described with reference to FIG. 1 again.

The semiconductor substrate 1 in the present embodiment may be a p-type substrate or an n-type substrate. However, if the semiconductor substrate 1 is an n-type substrate, the semiconductor region 18 is an n-type region as well as the first and second wells 16 a and 16 b. Therefore, in a chase where another circuit block is formed on the semiconductor substrate 1 in addition to a circuit block formed by the first and second transistors Tr₁ and Tr₂ and the like, and where the effect of noise between these circuit blocks needs to be eliminated or these blocks are controlled by different substrate bias controls, a p-type deep well region is formed under the first and second wells 16 a and 16 b. To form the deep well region, however, it is generally necessary to perform a highly-accelerated ion implantation. Therefore, the formation of the deep well region may increase the number of processes and the process cost. On the other hand, if the semiconductor substrate 1 is a p-type substrate, such a deep well region will be generally unnecessary. Therefore, if the semiconductor substrate 1 is a p-type substrate, the process of forming the deep well region can be omitted.

In the present embodiment, the positions of the first source region 11 a and the first drain region 12 a may be interchanged, and the positions of the second source region 11 b and the second drain region 12 b may be interchanged. In this case, the first and second drain regions 12 a and 12 b may be electrically connected by the interconnect 23 to form the inverter circuit.

In the present embodiment, as similar to connecting two TFETs of different conductivity types to form the inverter circuit, two or more TFETs of the same conductivity type may be cascode-connected to form a vertically-stacked circuit. In this case, the individual well regions of these TFETs may be replaced by a single well region as similar to the well region 16. In addition, the individual diffusion regions for substrate bias may be replaced by a single diffusion region as similar to the diffusion region 17.

(2) Method of Manufacturing Semiconductor Device of First Embodiment

FIGS. 5A to 6B are cross-sectional views showing a method of manufacturing the semiconductor device of the first embodiment.

First, as shown in FIG. 5A, the isolation insulators 5 a to 5 e are formed on the surface of the semiconductor substrate 1. For example, each of the isolation insulators 5 a to 5 e has a thickness of 200 to 300 nm in the Z direction. The isolation insulators 5 a to 5 e can be formed, for example, by forming isolation trenches on the surface of the semiconductor device 1 by lithography and etching, embedding a silicon oxide film in the isolation trenchs, and planarizing the surface of the silicon oxide film by chemical mechanical polishing (CMP).

A silicon oxide film (not shown) is then formed on a surface of a device region of the semiconductor substrate 1. For example, this silicon oxide film has a thickness of 10 nm or less.

A first ion implantation is then performed to form the first and second well regions 16 a and 16 b (FIG. 5A). For example, n-type impurities such as phosphorus (P) are used in the first ion implantation. In the first ion implantation of the present embodiment, the ion implantation energy is set at 500 keV, and the n-type impurity dose is set at 3.0×10¹³ cm⁻².

A second ion implantation is then performed to form the first and second channel regions 15 a and 15 b (FIG. 5A). For example, p-type impurities such as boron (B) are used in the second ion implantation. In the second ion implantation of the present embodiment, the ion implantation energy is set at 10 keV, and the p-type impurity dose is set at 1.5×10¹³ cm⁻². In the present embodiment, the threshold voltage of the first and second transistors Tr₁ and Tr₂ can be controlled by the second ion implantation.

Rapid thermal anneal (RTA) is then performed to activate the n- and p-type impurities implanted by the first and second ion implantations. Consequently, as shown in FIG. 5A, the first and second well regions 16 a and 16 b and the first and second channel regions 15 a and 15 b are formed in the semiconductor substrate 1. The first and second ion implantations may be performed in an order different from that in the present embodiment.

As shown in FIG. 5B, the first and second gate electrodes 3 a and 3 b are then formed on the first and second channel regions 15 a and 15 b via the first and second gate insulators 2 a and 2 b, respectively.

For example, the first and second gate insulators 2 a and 2 b and the first and second gate electrodes 3 a and 3 b can be formed by the following procedure. First, thermal oxidation or low pressure chemical vapor deposition (LPCVD) is performed to form a silicon oxide film for forming the first and second gate insulators 2 a and 2 b. For example, the silicon oxide film has a thickness of 0.5 to 6 nm. Next, a polysilicon layer for forming the first and second gate electrodes 3 a and 3 b is formed on the silicon oxide film. For example, the polysilicon layer has a thickness of 50 to 200 nm. The silicon oxide film and the polysilicon layer are then subjected to lithography and etching to form the first and second gate insulators 2 a and 2 b and the first and second gate electrodes 3 a and 3 b, respectively. Examples of the lithography include photolithography, X-ray lithography, electron beam lithography and the like. Examples of the etching include reactive ion etching (RIE) and the like.

In the present embodiment, pre-doping may be performed to implant n- and p-type impurities into the first and second gate electrodes 3 a and 3 b, respectively. For example, the n- and p-type impurities used in the pre-doping are phosphorus (P) and boron (B). In the pre-doping with the n-type impurities, the ion implantation energy is set at 5 keV, and the n-type impurity dose is set at 5.0×10¹⁵ cm⁻², for example. In the pre-doping with the p-type impurities, the ion implantation energy is set at 2.5 keV, and the p-type impurity dose is set at 5.0×10¹⁵ cm⁻², for example.

Thermal oxidation is then performed to form a post-oxidation film (not shown) on the side surfaces of the first and second gate electrodes 3 a and 3 b. For example, the post-oxidation film is a silicon oxide film.

An offset spacer film (not shown) is formed on the side surfaces of the first and second gate electrodes 3 a and 3 b via the post-oxidation film. For example, the offset spacer film is a silicon nitride film. The offset spacer film can be formed, for example, by forming a silicon nitride film with a thickness of 3 to 12 nm over the surface of semiconductor substrate 1 by LPCVD, and patterning the silicon nitride film by RIE.

A resist film (not shown) having an opening at a region where the first source region 11 a is to be formed is then formed on the semiconductor substrate 1. Next, a third ion implantation is performed to form the first diffusion region 13 a, and a fourth ion implantation is performed to form the third diffusion region 14 a (FIG. 6A). In the present embodiment, the third ion implantation is an angled ion implantation. The resist film is removed after the third and fourth ion implantations.

For example, n-type impurities such as arsenic (As) are used in the third ion implantation. In the third ion implantation of the present embodiment, the ion implantation energy is set at 40 keV, and the n-type impurity dose is set at 3.0×10¹³ cm⁻².

For example, the fourth ion implantation is performed using boron difluoride (BF₂). In the fourth ion implantation of the present embodiment, the ion implantation energy is set at 2 keV, and the p-type impurity dose is set at 1.0×10¹⁵ cm⁻².

A resist film (not shown) having an opening at a region where the second source region 11 b is to be formed is then formed on the semiconductor substrate 1. Next, a fifth ion implantation is performed to form the second diffusion region 13 b, and a sixth ion implantation is performed to form the fourth diffusion region 14 b (FIG. 6A). The resist film is removed after the fifth and sixth ion implantations.

For example, n-type impurities such as arsenic (As) are used in the fifth ion implantation. In the fifth ion implantation of the present embodiment, the ion implantation energy is set at 10 keV, and the n-type impurity dose is set at 3.0×10¹³ cm⁻².

For example, the sixth ion implantation is performed using boron difluoride (BF₂). In the sixth ion implantation of the present embodiment, the ion implantation energy is set at 2 keV, and the p-type impurity dose is set at 3.0×10¹³ cm⁻².

RTA is then performed to activate the n- and p-type impurities implanted by the third to sixth ion implantations. Consequently, as shown in FIG. 6A, the first and third diffusion regions 13 a and 14 a are formed in the vicinity of the first gate electrode 3 a, and the second and fourth diffusion regions 13 b and 14 b are formed in the vicinity of the second gate electrode 3 b. The third to sixth ion implantations may be performed in an order different from that in the present embodiment.

As shown in FIG. 6B, the first and second sidewall insulators 4 a and 4 b are then formed on the side surfaces of first and second gate electrodes 3 a and 3 b, respectively. For example, the first and second sidewall insulators 4 a and 4 b can be formed by forming a TEOS film over the surface of the semiconductor substrate 1, and patterning the TEOS film by an etchback process.

A seventh ion implantation is then performed to form the second source region 11 b, the first drain region 12 a, the fifth diffusion region 17 a, and the sixth diffusion region 17 b (FIG. 6B). For example, n-type impurities such as arsenic (As) and phosphorus (P) are used in the seventh ion implantation. In the seventh ion implantation of the present embodiment, the ion implantation energy is set at 20 keV for arsenic and at 5 keV for phosphorus, and the dose is set at 2.0×10¹⁵ cm⁻² for arsenic and at 2.0×10¹⁵ cm⁻² for phosphorus.

An eighth ion implantation is then performed to form the first source region 11 a and the second drain region 12 b (FIG. 6B). For example, p-type impurities such as boron (B) are used in the eighth ion implantation. In the eighth ion implantation of the present embodiment, the boron ion implantation energy is set at 2 keV, and the boron dose is set at 4.0×10¹⁵ cm⁻².

RTA is then performed to activate the n- and p-type impurities implanted by the seventh and eighth ion implantations. Consequently, as shown in FIG. 6B, the first source region 11 a and the first drain region 12 a are formed in the vicinity of the first gate electrode 3 a, and the second source region 11 b and the second drain region 12 b are formed in the vicinity of the second gate electrode 3 b. In addition, the fifth and sixth diffusion regions 17 a and 17 b are formed in the semiconductor substrate 1. The seventh and eighth ion implantations may be performed in an order different from that in the present embodiment.

Subsequently, silicide layers, inter layer dielectrics, plug layers, interconnect layers, a passivation layer and the like are formed on the semiconductor substrate 1. For example, the first and second plug layers 31 and 33, the first and second interconnect layers 32 and 34, the first to third inter layer dielectrics 41 to 43 and the like of FIG. 2 are formed. In this way, the semiconductor device of the present embodiment is manufactured.

When the semiconductor device of FIG. 3 is manufactured, the well region 16 is formed instead of the first and second well regions 16 a and 16 b in the step of FIG. 5A.

When the semiconductor device of FIG. 4 is manufactured, the isolation insulator 5 f is formed instead of the isolation insulators 5 b and 5 c in the step of FIG. 5A, and the well region 16 is formed instead of the first and second well regions 16 a and 16 b in the step of FIG. 5A. In this case, the diffusion region 17 is also formed instead of the fifth and sixth diffusion regions 17 a and 17 b in the step of FIG. 6B.

As described above, an n-type TFET of the first type is formed as the first transistor Tr₁, and a p-type TFET of the second type is formed as the second transistor Tr₂ in the present embodiment. Therefore, the present embodiment can provide a semiconductor device of a simple structure when a plurality of tunnel transistors (Tr₁ and Tr₂) are formed on the semiconductor substrate 1 to form a circuit.

(Second Embodiment)

FIG. 7 is a cross-sectional view showing a structure of a semiconductor device of a second embodiment.

The first transistor Tr₁ in the present embodiment is an n-type TFET of the second type in which the first source region 11 a and the first channel region 15 a have opposite conductivity types. In the present embodiment, the first source region 11 a, the first diffusion region 13 a, the first well region 16 a, and the fifth diffusion region 17 a are p-type regions, and the first drain region 12 a, the third diffusion region 14 a, and the first channel region 15 a are n-type regions. The first and third diffusion regions 13 a and 14 a function as a first source extension region for the first source region 11 a and as a pocket region for the first source region 11 a, respectively. The third diffusion region 14 a is formed on an upper side of the first diffusion region 13 a.

On the other hand, the second transistor Tr₂ in the present embodiment is a p-type TFET of the first type in which the second source region 11 b and the second channel region 15 b have the same conductivity type. In the present embodiment, the second source region 11 b, the fourth diffusion region 14 b, and the second channel region 15 b are n-type regions, and the second drain region 12 b, the second diffusion region 13 b, the second well region 16 b, and the sixth diffusion region 17 b are p-type regions. The fourth and second diffusion regions 14 b and 13 b function as a second source extension region for the second source region 11 b and as a halo region for the second source extension region, respectively. The second diffusion region 13 b is formed on lower and lateral sides of the fourth diffusion region 14 b.

The semiconductor substrate 1 in the present embodiment is an n-type substrate. Therefore, the semiconductor region 18 in the present embodiment is an n-type region.

The interconnect 23 in the present embodiment electrically connects the first and second drain regions 12 a and 12 b. The first and second transistors Tr₁ and Tr₂ in the present embodiment are electrically connected by the interconnect 23 to form an inverter circuit.

The interconnects 22 a and 22 b in the present embodiment are used to apply the ground voltage (=0 V) to the fifth and sixth diffusion regions 17 a and 17 b, respectively. According to the present embodiment, a reverse bias voltage can be applied to the pn junction between the first channel region 15 a and the first well region 16 a by applying the ground voltage to the fifth diffusion region 17 a. According to the present embodiment, a reverse bias voltage can also be applied to the pn junction between the second channel region 15 b and the second well region 16 b by applying the ground voltage to the sixth diffusion region 17 b.

The structures of the first and second modifications of the first embodiment (see FIGS. 3 and 4) are also applicable to the second embodiment.

(1) Method of Manufacturing Semiconductor Device of Second Embodiment

FIGS. 8A to 9B are cross-sectional views showing a method of manufacturing the semiconductor device of the second embodiment. In the description below, details of common features between the first and second embodiments will be omitted.

First, as shown in FIG. 8A, the isolation insulators 5 a to 5 e are formed on the surface of the semiconductor substrate 1.

The first ion implantation is then performed to form the first and second well regions 16 a and 16 b (FIG. 8A). For example, p-type impurities such as boron (B) are used in the first ion implantation. In the first ion implantation of the present embodiment, the ion implantation energy is set at 260 keV, and the p-type impurity dose is set at 2.0×10¹³ cm⁻².

The second ion implantation is then performed to form the first and second channel regions 15 a and 15 b (FIG. 8A). For example, n-type impurities such as arsenic (As) are used in the second ion implantation. In the second ion implantation of the present embodiment, the ion implantation energy is set at 80 keV, and the n-type impurity dose is set at 1.0×10¹³ cm⁻².

RTA is then performed to activate the n- and p-type impurities implanted by the first and second ion implantations.

As shown in FIG. 8B, the first and second gate electrodes 3 a and 3 b are then formed on the first and second channel regions 15 a and 15 b via the first and second gate insulators 2 a and 2 b, respectively.

Next, the third ion implantation is performed to form the first diffusion region 13 a, and the fourth ion implantation is performed to form the third diffusion region 14 a (FIG. 9A).

For example, the third ion implantation is performed using boron difluoride (BF₂). In the third ion implantation of the present embodiment, the ion implantation energy is set at 15 keV, and the p-type impurity dose is set at 3.0×10¹⁴ cm⁻².

For example, n-type impurities such as arsenic (As) are used in the fourth ion implantation. In the fourth ion implantation of the present embodiment, the ion implantation energy is set at 2 keV, and the n-type impurity dose is set at 2.4×10¹⁴ cm⁻².

Next, the fifth ion implantation is performed to form the second diffusion region 13 b, and the sixth ion implantation is performed to form the fourth diffusion region 14 b (FIG. 9A). In the present embodiment, the fifth ion implantation is an angled ion implantation.

For example, the fifth ion implantation is performed using boron difluoride (BF₂). In the fifth ion implantation of the present embodiment, the ion implantation energy is set at 20 keV, and the p-type impurity dose is set at 3.0×10¹³ cm⁻².

For example, n-type impurities such as arsenic (As) are used in the sixth ion implantation. In the sixth ion implantation of the present embodiment, the ion implantation energy is set at 1 keV, and the n-type impurity dose is set at 1.0×10¹⁵ cm⁻².

RTA is then performed to activate the n- and p-type impurities implanted by the third to sixth ion implantations.

As shown in FIG. 9B, the first and second sidewall insulators 4 a and 4 b are then formed on the side surfaces of the first and second gate electrodes 3 a and 3 b, respectively.

The seventh ion implantation is then performed to form the second source region 11 b and the first drain region 12 a (FIG. 9B). For example, n-type impurities such as arsenic (As) and phosphorus (P) are used in the seventh ion implantation. In the seventh ion implantation of the present embodiment, the ion implantation energy is set at 20 keV for arsenic and at 5 keV for phosphorus, and the dose is set at 2.0×10¹⁵ cm⁻² for arsenic and at 2.0×10¹⁵ cm⁻² for phosphorus.

The eighth ion implantation is then performed to form the first source region 11 a, the second drain region 12 b, the fifth diffusion region 17 a, and the sixth diffusion region 17 b (FIG. 9B). For example, p-type impurities such as boron (B) are used in the eighth ion implantation. In the eighth ion implantation of the present embodiment, the boron ion implantation energy is set at 2 keV, and the boron dose is set at 4.0×10¹⁵ cm⁻².

RTA is then performed to activate the n- and p-type impurities implanted by the seventh and eighth ion implantations.

Subsequently, the silicide layers, the inter layer dielectrics, the plug layers, the interconnect layers, the passivation layer and the like are formed on the semiconductor substrate 1. For example, the first and second plug layers 31 and 33, the first and second interconnect layers 32 and 34, the first to third inter layer dielectrics 41 to 43 and the like of FIG. 2 are formed. In this way, the semiconductor device of the present embodiment is manufactured.

As described above, an n-type TFET of the second type is formed as the first transistor Tr₁, and a p-type TFET of the first type is formed as the second transistor Tr₂ in the present embodiment. Therefore, the present embodiment can provide a semiconductor device of a simple structure when a plurality of tunnel transistors (Tr₁ and Tr₂) are formed on the semiconductor substrate 1 to form a circuit.

(Third Embodiment)

FIG. 10 is a circuit diagram showing a circuit configuration of a semiconductor device of a third embodiment.

The semiconductor device of FIG. 10 includes the semiconductor substrate 1 (not shown), first transistors 101 a to 101 c formed on the semiconductor substrate 1, and second transistors 102 a to 102 c formed on the semiconductor substrate 1. The first and second transistors 101 a to 102 c form a NAND circuit.

Each of the first transistors 101 a to 101 c is an n-type TFET. The first transistors 101 a to 101 c are connected in series (cascode connection). The source S of the first transistor 101 a is electrically connected to a ground voltage (=0 V), and the drain D of the first transistor 101 a is electrically connected to the source S of the first transistor 101 b. The drain D of the first transistor 101 b is electrically connected to the source S of the first transistor 101 c. The drain D of the first transistor 101 c is electrically connected to the output part 103 of the NAND circuit.

Each of the second transistors 102 a to 102 c is a p-type TFET. The second transistors 102 a to 102 c are connected in parallel and electrically connected to the first transistors 101 a to 101 c. The sources S of the second transistors 102 a to 102 c are electrically connected to a power supply voltage (=V_(DD)). The drains D of the second transistors 102 a to 102 c are electrically connected to the drain D of the first transistor 101 c and the output part 103 of the NAND circuit.

The first transistors 101 a to 101 c of the present embodiment have the same structure as the first transistor Tr₁ of FIG. 1. Therefore, each of the first transistors 101 a to 101 c of the present embodiment is the first type TFET in which the first source region 11 a and the first channel region 15 a have the same conductivity type. In each of the TFETs, when the power supply voltage is applied to the interconnect 22 a and the fifth diffusion region 17 a, the reverse bias voltage can be applied to the pn junction between the first channel region 15 a and the first well region 16 a.

On the other hand, the second transistors 102 a to 102 c of the present embodiment have the same structure as the second transistor Tr₂ of FIG. 1. Therefore, each of the second transistors 102 a to 102 c of the present embodiment is the second type TFET in which the second source region 11 b and the second channel region 15 b have opposite conductivity types. In each of these TFETs, when the power supply voltage is applied to the interconnect 22 b and the sixth diffusion region 17 b, the reverse bias voltage can be applied to the pn junction between the second channel region 15 b and the second well region 16 b.

Alternatively, the first transistors 101 a to 101 c of the present embodiment may have the same structure as the first transistor Tr₁ of FIG. 3, and the second transistors 102 a to 102 c of the present embodiment may have the same structure as the second transistor Tr₂ of FIG. 3. In other words, the first and second well regions 16 a and 16 b of these transistors may be replaced by the well region 16.

Similarly, the first transistors 101 a to 101 c of the present embodiment may have the same structure as the first transistor Tr₁ of FIG. 4, and the second transistors 102 a to 102 c of the present embodiment may have the same structure as the second transistor Tr₂ of FIG. 4. In other words, the fifth and sixth diffusion regions 17 a and 17 b of these transistors may be replaced by the diffusion region 17.

In the present embodiment, the sources S of the first transistors 101 b and 101 c are electrically floating. Therefore, if a forward bias voltage is applied to the pn junction between the first channel region 15 a and the first well region 16 a in these transistors, a leakage current can flow between these regions. However, according to the present embodiment, a reverse bias voltage can be applied to the pn junction so that the leakage current can be prevented from flowing between these regions. The reverse bias voltage can be applied by using the fifth diffusion region 17 a and the interconnect 22 a in these transistors.

The first transistors 101 a to 101 c of the present embodiment may have the same structure as the first transistor Tr₁ of FIG. 7, and the second transistors 102 a to 102 c of the present embodiment may have the same structure as the second transistor Tr₂ of FIG. 7. In this case, the first transistors 101 a to 101 c of the present embodiment are the second type TFETs, and the second transistors 102 a to 102 c of the present embodiment are the first type TFETs.

(Fourth Embodiment)

FIG. 11 is a circuit diagram showing a circuit configuration of a semiconductor device of a fourth embodiment.

The semiconductor device of FIG. 11 includes the semiconductor substrate 1 (not shown), first transistors 201 a to 201 c formed on the semiconductor substrate 1, and second transistors 202 a to 202 c formed on the semiconductor substrate 1. The first and second transistors 201 a to 202 c form a NOR circuit.

Each of the first transistors 201 a to 201 c is an n-type TFET. The first transistors 201 a to 201 c are connected in parallel and electrically connected to the second transistors 202 a to 202 c. The sources S of the first transistors 201 a to 201 c are electrically connected to a ground voltage (=0 V). The drains D of the first transistors 201 a to 201 c are electrically connected to the drain D of the second transistor 202 c and the output part 203 of the NOR circuit.

Each of the second transistors 202 a to 202 c is a p-type TFET. The second transistors 202 a to 202 c are connected in series (cascode connection). The source S of the second transistor 202 a is electrically connected to a power supply voltage (=V_(DD)), and the drain D of the second transistor 202 a is electrically connected to the source S of the second transistor 202 b. The drain D of the second transistor 202 b is electrically connected to the source S of the second transistor 202 c. The drain D of the second transistor 202 c is electrically connected to the output part 203 of the NOR circuit.

The first transistors 201 a to 201 c of the present embodiment have the same structure as the first transistor Tr₁ of FIG. 1, 3, or 4. Therefore, the first transistors 201 a to 201 c of the present embodiment are the first type TFETs.

On the other hand, the second transistors 202 a to 202 c of the present embodiment have the same structure as the second transistor Tr₂ of FIG. 1, 3, or 4. Therefore, the second transistors 202 a to 202 c of the present embodiment are the second type TFETs.

Alternatively, the first transistors 201 a to 201 c of the present embodiment may have the same structure as the first transistor Tr₁ of FIG. 7, and the second transistors 202 a to 202 c of the present embodiment may have the same structure as the second transistor Tr₂ of FIG. 7. In this case, the first transistors 201 a to 201 c of the present embodiment are the second type TFETs, and the second transistors 202 a to 202 c of the present embodiment are the first type TFETs.

In this case, the sources S of the second transistors 202 b and 202 c are electrically floating. Therefore, if a forward bias voltage is applied to the pn junction between the second channel region 15 b and the second well region 16 b in these transistors, a leakage current can flow between these regions. However, according to the present embodiment, a reverse bias voltage can be applied to the pn junction so that the leakage current can be prevented from flowing between these regions. The reverse bias voltage can be applied by using the sixth diffusion region 17 b and the interconnect 22 b in these transistors.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the devices described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

The invention claimed is:
 1. A semiconductor device comprising: a semiconductor substrate; a first transistor of a first conductivity type disposed on the semiconductor substrate; and a second transistor of a second conductivity type disposed on the semiconductor substrate; wherein the first transistor comprises: a first gate electrode disposed on the semiconductor substrate via a first gate insulator; a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode; and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region; wherein the second transistor comprises: a second gate electrode disposed on the semiconductor substrate via a second gate insulator; a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode; and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region; and wherein the semiconductor device further comprises: a well region disposed under the first and second channel regions to be in contact with the first and second channel regions, and having an opposite conductivity type of the first and second channel regions; and one or more interconnects configured to apply reverse bias voltages to pn junctions between the well region and the first and second channel regions, the interconnects being disposed above the semiconductor substrate.
 2. A semiconductor device comprising: a semiconductor substrate; a first transistor of a first conductivity type disposed on the semiconductor substrate; and a second transistor of a second conductivity type disposed on the semiconductor substrate; wherein the first transistor comprises: a first gate electrode disposed on the semiconductor substrate via a first gate insulator; a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode; and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region; wherein the second transistor comprises: a second gate electrode disposed on the semiconductor substrate via a second gate insulator; a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode; and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region; and wherein the semiconductor device further comprises: a well region disposed under the first and second channel regions to be in contact with the first and second channel regions, and having an opposite conductivity type of the first and second channel regions; and a diffusion region disposed to electrically connect an interconnect above the semiconductor substrate and the well region, and having the same conductivity type as the well region.
 3. The device of claim 1, further comprising an isolation insulator disposed in contact with the first and second drain regions, or disposed in contact with the first and second source regions.
 4. The device of claim 1, wherein the semiconductor substrate has an opposite conductivity type of the well region.
 5. A semiconductor device comprising: a semiconductor substrate; a first transistor of a first conductivity type disposed on the semiconductor substrate; and a second transistor of a second conductivity type disposed on the semiconductor substrate; wherein the first transistor comprises: a first gate electrode disposed on the semiconductor substrate via a first gate insulator; a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode; and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region; wherein the second transistor comprises: a second gate electrode disposed on the semiconductor substrate via a second gate insulator; a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode; and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region; and wherein the semiconductor device further comprises: a first well region disposed under the first channel region to be in contact with the first channel region, and having an opposite conductivity type of the first channel region; and a second well region disposed under the second channel region to be in contact with the second channel region, and having an opposite conductivity type of the second channel region.
 6. The device of claim 5, further comprising: a first interconnect configured to apply a reverse bias voltage to a pn junction between the first channel region and the first well region, the first interconnect being disposed above the semiconductor substrate; and a second interconnect configured to apply a reverse bias voltage to a pn junction between the second channel region and the second well region, the second interconnect being disposed above the semiconductor substrate.
 7. The device of claim 5, further comprising: a first region disposed to electrically connect a first interconnect above the semiconductor substrate and the first well region, and having the same conductivity type as the first well region; and a second region disposed to electrically connect a second interconnect above the semiconductor substrate and the second well region, and having the same conductivity type as the second well region.
 8. The device of claim 5, wherein the semiconductor substrate has an opposite conductivity type of the first and second well regions.
 9. The device of claim 5, further comprising: a first source extension region of the second conductivity type disposed in contact with the first source region; a second source extension region of the first conductivity type disposed in contact with the second source region; and a pocket region of the second conductivity type disposed in contact with the second source region, and disposed on an upper side of the second source extension region.
 10. The device of claim 9, further comprising a halo region of the first conductivity type disposed in contact with the first source region, and disposed on lower and lateral sides of the first source extension region.
 11. The device of claim 5, further comprising: a first source extension region of the second conductivity type disposed in contact with the first source region; a second source extension region of the first conductivity type disposed in contact with the second source region; and a pocket region of the first conductivity type disposed in contact with the first source region, and disposed on an upper side of the first source extension region.
 12. The device of claim 11, further comprising a halo region of the second conductivity type disposed in contact with the second source region, and disposed on lower and lateral sides of the second source extension region.
 13. The device of claim 5, further comprising an interconnect disposed above the semiconductor substrate to electrically connect the first and second drain regions.
 14. A semiconductor device comprising: a semiconductor substrate; first transistors of a first conductivity type disposed on the semiconductor substrate; and second transistors of a second conductivity type disposed on the semiconductor substrate; wherein the first transistors are connected in series and the second transistors are connected in parallel and electrically connected to the first transistors, or the second transistors are connected in series and the first transistors are connected in parallel and electrically connected to the second transistors; wherein each of the first transistors comprises: a first gate electrode disposed on the semiconductor substrate via a first gate insulator; a first source region of the second conductivity type and a first drain region of the first conductivity type disposed to sandwich the first gate electrode; and a first channel region of the first or second conductivity type disposed between the first source region and the first drain region; and wherein each of the second transistors comprises: a second gate electrode disposed on the semiconductor substrate via a second gate insulator; a second source region of the first conductivity type and a second drain region of the second conductivity type disposed to sandwich the second gate electrode; and a second channel region disposed between the second source region and the second drain region and having the same conductivity type as the first channel region.
 15. The device of claim 14, further comprising a well region disposed under the first and second channel regions to be in contact with the first and second channel regions, and having an opposite conductivity type of the first and second channel regions.
 16. The device of claim 15, further comprising one or more interconnects configured to apply reverse bias voltages to pn junctions between the well region and the first and second channel regions, the interconnect being disposed above the semiconductor substrate.
 17. The device of claim 15, further comprising a diffusion region disposed to electrically connect an interconnect above the semiconductor substrate and the well region, and having the same conductivity type as the well region.
 18. The device of claim 14, further comprising: a first well region disposed under the first channel region to be in contact with the first channel region, and having an opposite conductivity type of the first channel region; and a second well region disposed under the second channel region to be in contact with the second channel region, and having an opposite conductivity type of the second channel region. 